The present invention relates generally to the field of tunable filters. More particularly, the present invention relates to tunable filter covering an appreciable frequency range while maintaining a constant bandwidth over that range.
Tunable filters offer communications service providers flexibility and scalability never before accessible. A single tunable filter can replace several fixed filters covering adjacent frequencies. This versatility provides transceiver front end RF tunability in real time applications and decreases deployment and maintenance costs through software controls and reduced component count. Tunable filters, although typically narrow band, can cover a larger frequency band than fixed filters by tuning over a wide range. Narrowband filters at the front end are appreciated from a systems point of view because they provide better selectivity and help reduce interference from nearby transmitters.
There are many potential uses for miniaturized, low-cost, tunable filters. Examples include software reconfigurable radios, mobile communications, and wideband radar systems. However, traditional varactor or switched capacitor tuned filter approaches have limitations caused by insertion loss and/or bandwidth variation. For example, stepped impedance resonant filters, in which the resonant frequency is tuned by direct physical transmission line adjustment, use external and internal lumped element networks to vary the coupling across the tuning range in order to eliminate bandwidth variation. However this approach requires active gain elements to compensate for the loss variation. In another example, comb-line and inter-digital filters, in which the resonant frequency is tuned by indirect capacitive loading of the resonant transmission line elements, use switchable coupling capacitors along the length of the resonator lines in order to eliminate the bandwidth variation. However, these types of filters tend to be complicated.
Current methods of actively turning filters require that the coupling between resonators be tuned as the resonant frequency of resonators is tuned in order to achieve constant bandwidth across the tuning range. This coupling between resonators, whether magnetic or electric, is very small and very sensitive. Accordingly, it is challenging if not prohibitive due to manufacturing and yield costs to design tunable filters having a dynamic coupling between resonators.
What is needed is an actively tuned filter in which an inter-resonator coupling of resonators decreases as the turning frequency is increased thereby maintaining constant bandwidth. What is further needed is such a filter where only the resonant frequency of the resonators is actively tuned and complicated internal coupling networks are not required.
It would be desirable to provide a system and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments which fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.